This invention relates to stabilizing halogenated polymers and to an improved method for stabilization thereof. More particularly, the invention relates to thermal stabilization of halogenated polymer resins that are normally processed at elevated temperatures into formed articles. In particular, the invention relates to thermal stabilization of such resins with certain synthetic crystalline aluminosilicates or zeolite molecular sieves prepared specifically for this purpose.
Halogen containing polymers, such as polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC) and other chlorine, fluorine, or bromine containing polymers which are formed above about 150.degree. C. tend to degrade and discolor unless stabilized, even when held at these temperatures for the relatively short period of time required for processing. When this occurs, the polymer is either unusable or a high rejection rate for produced articles results, and the ability to reuse scrap material from the formation process is seriously impaired.
Subsequent to processing such resins, articles made from such halogenated polymers, unless stabilized and properly pigmented, also tend to deteriorate, become brittle and crack or shatter upon prolonged exposure to sunlight or other ultraviolet rays, thereby becoming useless for their intended purpose.
These resins in their unstabilized form also tend to plate out on processing equipment, for example on dies and mill or calendar rolls, as the material is being formed into a finished article. This causes imperfections in articles made from such resins and tends to require frequent cleaning of processing equipment used for handling such materials, resulting in inefficiencies in production as well. For example, processing of unstabilized PVC at elevated temperatures rapidly results in degradation whose symptoms are discoloration, elimination of hydrogen chloride, and irreversible adhesion to the processing equipment surfaces.
While all mechanisms for these instabilities are not precisely known, the thermal degradation and perhaps the other indicated forms of instability of halogenated polymers, particularly polyvinyl chloride (PVC) or chlorinated polyvinyl chloride (CPVC), is manifested in the evolution of HCl. It is widely known that HCl catalyzes further degradation. Prevention of this degradation requires keeping HCl at very low concentrations and/or neutralizing it during processing.
Traditional methods for stabilizing halogenated polymers such as PVC have focused on the use of various inorganic, organometallic and organic stabilizers. Inorganic stabilizers that have been used include for example dibasic, tribasic, and tetrabasic lead sulfate; dibasic lead phosphite; and white lead. Organometallic stabilizers commonly used include organic adducts of such heavy metals as barium, cadmium, lead, tin, magnesium, antimony, and/or zinc, frequently in admixture with other co-stabilizers and other conventional additives. Organic stabilizers that have been used include for example, calcium soaps, polyhydric esters of various fatty acids, phosphites, thioesters, beta-diketones and the like, alone or in combination with such organometallic compounds as stabilizers for such resins.
Aluminosilicates or zeolite molecular sieves have also been suggested for use as stabilizers for PVC. For example, U.S. Pat. No. 3,245,946 discloses the use of activated Zeolite A as a stabilizer for PVC resin. And U.S. Pat. No. 4,000,100 discloses the use of an unactivated Zeolite 3A, 4A, or 5A molecular sieve in combination with a conventional organometallic or organic stabilizer mixture. It also known to utilize a complex system of primary and secondary stabilizers, including as one component of the stabilization system, a powdered, crystalline, synthetic hydrous aluminosilicate having a water content in the range of 13 to 25% as water of crystallization, as disclosed in U.S. Pat. No. 4,590,233
Among the prior art utilizing zeolites or aluminosilicates as part of a stabilization system for PVC resin, focus has been on such factors as the level of water of crystallization, and/or on pore size of the aluminosilicate. Preferred particle size ranges of zeolites are also described. Presumably certain size ranges give optimal dispersion and enhanced physical properties such as tensile strength and modulus as is known with other solid polymer additives such as calcium carbonate.
However, we are not aware of any prior art disclosure which establishes or suggests a direct correlation between reduction in particle size and enhancement of thermal stability. Nor are aware of any significance attached in prior art disclosures to the distinction made with respect to this invention between particle size and crystallite size. Conventional zeolites consist of small cubic or prismatic crystallites and/or other geometric forms such as rhombic, dodecahedral, spherulites, octahedral etc., and combinations and intergrowths thereof, that agglomerate into particles. The degree of agglomeration and/or inter-growth determines the particle size distribution, which is typically determined by a light scattering or other spectroscopic technique, whereas crystallite size is virtually independent of particle size and is typically determined by scanning electron microscopy images. As an example of this distiniction, zeolite 4A and faujasite-type zeolites currently offered on the market typically have a crystallite size of about 1.0 to 5.0 microns with a mean particle size of about 3.0 to 10.0 or more microns. Moreover, while the prior art does recognize methods for reducing particle size, it does not discuss preparative procedures for the small crystallite zeolites as claimed in the present application.
While the use of factors such as complex stabilization systems and activation of zeolites has resulted in substantial improvement in the ability of aluminosilicates to stabilize PVC resins, still further improvements are required, particularly in the level of thermal stabilization for PVC, CPVC, and other halogenated polymers.
Accordingly, it is the primary object of this invention to provide an improvement in the thermal stability of halogenated polymer molding resins such as PVC, CPVC, and other halogenated polymers utilized for forming articles at elevated temperature.